I would buy an insulated steel entry door and bring it to a place that fabricates stainless steel and have them just apply a sheet over it. They would need to have their tolerances pretty close so that if they lipped it over the edge that it wouldn't add too much to the dimensions of the door (width and height) and interfere with the correct closing. Or, they could use a thin shaped channel around the edges (mounted on the face) into which the sheet edge would terminate. Basically it comes down to attaching a piece of sheet metal (stainless) to the door. I use glue (epoxy or construction adhesive and sometimes edge channels and/or decorative brass screws with finish washers or special knurled heads, etc..